I have previously devised novel and improved scissor frame hoists employed on dump trucks for the purpose of stabilizing the movement of the dump body as it is elevated and tilted rearwardly by a hydraulic cylinder lift mechanism. For example, reference is made to my U.S. Pat. Nos. 3,809,429 and 4,148,528. In my more recent U.S. Pat. No. 4,148,528, the scissor frame is broadly characterized by comprising upper and lower legs which are pivotally interconnected in end-to-end relation to one another via an elbow pivot which is offset from the legs. Opposite ends of the upper and lower leg members are respectively connected in pivotal relation to the underside of the dump body and to the frame or chassis of the truck.
A criterion of the scissor frame as described in my hereinbefore referred to U.S. Pat. No. 4,148,528 is that the upper and lower leg members are formed as flat rectangular tubes and are dimensioned in length such that in the retracted or lowered position they are capable of extending through the space formed between longitudinal beams of the dump body and chassis as well as to permit the hydraulic cylinder lift members to extend along either side of the scissor frame and similarly to fit within the space formed between the scissor frame and longitudinal base beam members of the truck chassis. In order to lift the dump body, hydraulic pressure is applied to the lift cylinders thereby causing the dump body to be elevated and tilted about a rear hinge axis with the scissor frame resisting any tendency of the dump body to twist or sway as it is lifted. The cylinders may either be single or double-acting and are dimensioned in length so as to extend the dump body to a raised position necessary for the load to be dumped therefrom. The cylinders customarily will reach their limit of extension before the leg members of the scissor frames are fully straightened. As a result, the cylinders must absorb any lift or impact forces once the center of gravity of the load in the dump body is shifted to a degree sufficient to impart continued lift forces to the cylinders. Correspondingly, with loads which are difficult to remove from the dump body, it is customary to alternately raise and lower the dump body through a succession of rapid cylinder strokes. In either case, extremely heavy impact loads are placed on the cylinders which can result in damage to the cylinders and externally cause the cylinder seals to leak.
It is therefore desirable to devise a novel and improved scissor frame hoist assembly which will maintain the requisite stability and torsion resistance in lifting a dump body while at the same time is capable of effectively absorbing impact loads and forces applied by the dump body as it reaches its limit of upward extension in tilting so as to remove such forces from the hydraulic cylinders.
Further, a novel and improved self-centering support for the lower ends of the cylinders enables greater freedom of rotation of the cylinders and minimizes deflection of the saddle support assembly under large cylinder thrust loads. In connection with the latter, greater freedom of rotation of the cylinder together with reduced deflection of the saddle assembly itself is achieved by placing mutually perpendicular pivotal and rotational axes of the saddle assembly in close proximity to one another.